Optical fiber gain medium with evanescent filtering

ABSTRACT

An optical fiber used as the active amplifying medium in a fiber laser is arranged to have a high insertion loss at an undesired frequency, while retaining a low insertion loss at a desired lasing frequency. In one embodiment, loss at a Raman-shifted frequency is introduced by using an optical fiber which has multiple claddings with an index profile that includes an elevated index region located away from the core, but within the evanescent coupling region of the core. A distributed loss, which can be enhanced by bending, is produced at the Raman frequency which effectively raises the threshold at which Raman scattering occurs in the fiber and therefore results in a frequency-selective fiber. In another embodiment, an absorbing layer is placed around the core region. The absorbing layer is chosen to have a sharp absorption edge so that it absorbs highly at the Raman-shifted wavelength, but minimally at the desired lasing wavelength. In still another embodiment, the optical fiber is constructed with a core with long period gratings formed therein. The gratings are fabricated with a periodicity selected to provide a relatively high insertion loss at the Raman frequency while simultaneously providing a relatively low insertion loss at the lasing frequency. In accordance with yet another embodiment, a bend loss technique is used to suppress amplified spontaneous emission at an unwanted wavelength due to a competing atomic energy level system in a fiber laser.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under ContractNumber N61331-93-C-0061 awarded by ARPA. The government has certainrights in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.08/819,689, filed on Mar. 17, 1997 now U.S. Pat. No. 5,892,615.

FIELD OF THE INVENTION

This invention relates generally to fiber lasers and, more particularly,to high-power fiber lasers.

BACKGROUND OF THE INVENTION

Diode lasers are often used to pump erbium single-mode fiber amplifiers;however, a single diode laser typically generates only a relativelysmall amount of pumping power. Consequently, an array of diode lasers,or a laser bar, is conventionally used to generate a pump power levelwhich is relatively large when compared with the pump power levelprovided by a single diode laser. The output beam produced by an arrayof laser diodes is highly multi-mode and thus not suitable for launchingdirectly into a single mode fiber core. Therefore, in order toeffectively couple a diode laser array to a single mode fiber core, atechnique commonly referred to as "cladding pumping" is used. In acladding pump technique, a single-mode core is surrounded by amulti-mode cladding layer which, in turn, is surrounded by an outermostcladding layer. A relatively high-power multimode pumping signallaunched into the cladding from a diode array is substantially confinedand guided within the multi-mode cladding layer. As the pumping energypropagates along the multi-mode cladding layer criss-crossing the dopedfiber core, the energy is absorbed by the single-mode core. The absorbedmulti-mode power is converted into a single-mode laser emission withinthe core. For many applications, this is an effective technique forsupplying a relatively high-power pumping signal to a single-mode fiberlaser.

However, one factor which limits the output power characteristic of acladding pumped fiber laser is the conversion of a portion of the laseroutput signal from the desired lasing frequency to an unwanted so-called"Raman frequency." This conversion occurs by a process known asstimulated Raman scattering which shifts the desired output frequency tothe first Stokes frequency of the fiber core. In optical fibersmanufactured from silica, the first Stokes frequency corresponds to awavelength of approximately 450 cm⁻¹ or about 60 nanometers (nm) in theregion of 1100 nm. Raman scattering is power-related and becomessignificant when power levels increase above a threshold. The powerlimitation imposed by Raman scattering is more severe in pulsed fiberlaser systems. With the effective area of typical cladding-pumped fibersand fiber lengths of about 50 meters, Raman scattering becomessignificant at output power levels typically in the range of a few tensof watts. When Raman shifting occurs, the shifted wavelengths are alsoamplified in the laser oscillator thereby diverting the pumping energyalso to the Raman-shifted wavelength output. Output at the desiredlasing wavelength is therefore effectively limited.

A similar problem occurs when the laser gain at a desired lasingwavelength is less than the laser gain at another wavelength. In thiscase, amplified spontaneous emission (ASE) may occur at the otherwavelength and prevent the laser from operating at the desiredwavelength or require an increase in the pumping intensity in order togenerate a desired power output. For example, it may be desirable tooperate a neodymium (Nd) fiber laser comprised of a Nd-doped double cladfiber at a laser wavelength of 940 nm. However, neodynium is athree-level system at 940 nm and the amplifier gain of the fiber isusually higher at 1060 nm where Nd is a four-level system. Consequently,operation at a wavelength of 940 nm requires a relatively high pumpintensity due to the presence of the competing energy level system.

It is therefore desirable to control the output power characteristics ofcladding pumped fiber lasers.

It is further desirable to suppress unwanted wavelengths in claddingpumped fiber lasers.

It is further desirable to reduce the effective pumping of a shiftedwavelength output due to Raman scattering.

SUMMARY OF THE INVENTION

In accordance with the present invention, the optical fiber employed asthe active amplifying medium in a fiber laser is arranged to have a highinsertion loss at the undesired frequency, while retaining a lowinsertion loss at the desired lasing frequency. In one embodiment, lossat an undesired frequency is introduced by using an optical fiber whichhas multiple claddings with an index profile which may include anelevated index region located in close spatial proximity to the core,specifically, within the evanescent coupling region of the core. Thisraised index section produces a distributed loss at the undesiredfrequency which is several orders of magnitude higher than the loss inan unmodified fiber, but no, or minimal, loss is introduced at thedesired lasing frequency. This approach effectively raises the thresholdat which, for example, Raman scattering occurs in the fiber and,therefore, results in a frequency-selective fiber. When such a fiber isused as the active medium in a laser, the output power can beconsiderably higher before Raman shifting occurs.

In another embodiment, an absorbing layer is placed around the coreregion. The absorbing layer is chosen to have a sharp absorption edge sothat it absorbs highly at the undesired wavelength, but minimally at thedesired lasing wavelength. Such a structure also produces a fiber with adistributed loss at the undesired wavelength.

In still another embodiment, the optical fiber is constructed with acore with long period gratings formed therein. The gratings arefabricated with a periodicity selected to provide a relatively highinsertion loss at the undesired frequency while simultaneously providinga relatively low insertion loss at the lasing frequency. The long periodgratings act as discrete loss elements to undesired wavelength signalspropagating in the optical fiber. In some applications, it may bepreferable to construct the fiber core with a region having a pluralityof long period gratings formed along the fiber length. This approach canbe realized in conventional optical fibers by conventional dopingtechniques.

In accordance with yet another embodiment, a bend loss technique is usedto suppress the undesired wavelengths. For example, bending loss can beused to suppress amplified spontaneous emission (ASE) at an unwanted,longer wavelength due to a competing atomic energy level system in afiber laser. The technique includes continuously bending the fiber witha selected bend radius and pumping the fiber with a pump signal. Whenthe bend radius is selected properly, light at the unwanted wavelengthradiates out of the core. The gain of the fiber at the unwantedwavelength is inhibited to prevent the occurrence of ASE at the unwantedwavelength, allowing the fiber to operate as a laser in the desiredlasing wavelength.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing features of the invention, as well as the invention itselfmay be more fully understood from the following detailed description ofthe drawings, in which:

FIG. 1 is a cross-sectional view of an optical fiber constructed inaccordance with the principles of the invention;

FIG. 1A is a graphic illustration of the loss characteristic versuswavelength with, and without, bending of the fiber.

FIG. 1B is a plot of the index of refraction versus distance along across-sectional axis of the optical fiber shown in FIG. 1;

FIG. 2 is a cross-sectional view of an optical fiber having anabsorptive ring; and

FIG. 3 is a schematic diagram of an optical fiber having a single longperiod grating.

FIG. 4 is a schematic diagram of an optical fiber having a plurality ofdistributed long period gratings.

FIG. 5 is a diagrammatical view of a fiber laser system with bendingloss introduced a particular wavelength to suppress ASE from a competinglevel system.

FIG. 5A is a graphic plot of insertion loss vs. wavelength.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, an optical fiber 10 includes an innermulti-core/cladding section, as shown, comprising a single-mode core 12which, in this particular embodiment, is fabricated from silica glasshaving an index of refraction η_(core) typically around of about 1.47and a radius R₁ typically of around 2-5 μm and doped with appropriaterare earth ions. Such rare earth ions constitute the active element fora fiber laser or fiber amplifier. For example, core 12 may be doped witherbium (Er) ions (typically denoted Er³⁺), in which case optical fiber10 exhibits gain at a wavelength typically around 1.5 μm. Also, the coremay be doped with neodymium (Nd) or ytterbium (Yb) or combinations ofEr, Nd or Yb, such as, for example, Er:Yb or Nd:Yb.

The single inner mode core 12 is surrounded by a multi-mode claddingregion 14 having a circularly shaped cross-section. Cladding region 14encompasses the entire longitudinal extent of core 12. Cladding region14 may be provided, for example, from undoped silica having an index ofrefraction less than the index of refraction of the single mode core 12.In one particular embodiment, for example, the index of refractionη_(clad1) of cladding 14 may be about 1.44 and the outer radius R₂ ofcladding 14 (layer thickness) is typically about 2-5 μm.

In turn, the first cladding layer 14 is surrounded by a second claddinglayer 16. The second cladding layer is also provided having a circularlyshaped cross-section and is formed from a suitable polymer, or from aconventional low-index glass. The index of refraction η_(clad2) of thesecond cladding layer 16 is selected to be greater than the index ofrefraction η_(clad1) of the first cladding layer 14. For example, in thecase where the index of refraction η_(core) of core 12 is 1.47 and theindex of refraction η_(clad1) of cladding layer 14 is 1.44, the index ofrefraction η_(clad2) of cladding layer 16 may be about 1.45. In thisparticular embodiment, the outer radius R₃ (layer thickness) of thesecond cladding layer 16 is typically about 2-5 μm.

While not shown in FIG. 1, second cladding 16 is surrounded by a pumpcladding that has a diameter of around 250 μm, for example, forreceiving pump light from the input end of fiber 10 for pumping rareearth doped core 12. The diameter of the pump cladding is proportionallymuch greater than the diameter of the core/cladding section of fiber 10.For example, the diametrical extent of the core/cladding section offiber 10 may be only about 4 μm to 10 μm compared to the 250 μm pumpcladding. As known to those skilled in this art, the pump cladding mayhave a cross-sectional shape which differs from the core/claddingcross-section shape of fiber 10. For example, the cross-sectional shapeof the pump cladding may be square, rectangular or elliptical. The pumpcladding would also be surrounded by an outer light-confining,protective cladding.

It should be noted that the relative values, rather than the absolutevalues of the refractive index of each of the layers 12-18 are ofparticular importance. The particular refractive index of each of layers12-18 may be selected based upon a variety of factors including, but notlimited to, a selected lasing wavelength, cladding material, dopants,type of fiber and other factors. Likewise, the outer radius of each ofthe layers may be selected based upon a variety of factors including butnot limited to a selected laser wavelength, material and dopants.

This fiber produces a distributed loss at the undesired Raman-shiftedfrequencies. This loss can be enhanced by bending the fiber at anappropriate bend radius. Consequently, for the practice of thisinvention, the use of fibers highly sensitive to bending are preferred,such as W-fibers or QC fibers. FIG. 1 is an example of the W-fiber. QCfibers would also work, but W-fibers are more sensitive to bending thanQC fibers and, consequently will provide a sharper edge cutoff forhigher, unpreferred frequencies. These fibers are sensitive to bendingand, with appropriate bending, may provide enhanced loss at higherwavelengths. As an example, if the wavelength of operation desired in afiber laser has a peak at 1110 nm and the unpreferred higher wavelengthcaused by Raman scattering has a peak at about 1180 nm, then arelatively sharp cutoff for Raman-shifted higher wavelengths can beprovided by constructing the fiber in accordance with FIG. 1 and bendingthe fiber. These modifications cause the Raman-shifted higher wavelengthat 1180 nm to radiate out of the fiber. This is illustrated in FIG. 1Awherein it can be seen that the loss from fiber 10 increases withwavelength as illustrated by curve 11. This invention takes advantage ofthat loss through loss enhancement of Raman-shifted higher wavelength of1180 nm by bending fiber 10 to provide a relatively sharp cutoff edge asillustrated by curve 13 in FIG. 1A.

Referring now to FIG. 1B, a generalized plot of the cross-sectionalindex of refraction of the core/cladding section of optical fiber 10constructed in accordance with the principles of the invention is shown.FIG. 1B is for illustrative purposes and the radii and the refractiveindex relationships illustrated are not necessarily drawn to relativescale. As shown in plot, the core 12 (FIG. 1) has an index of refractionwhich is higher than the index of refraction of cladding layers 14 and16 (FIG. 1). The core 12 is surrounded by the first cladding layer withan index of η_(clad1). The first cladding layer is, in turn, surroundedby the second cladding layer with an elevated index of refection,η_(clad2). The radii R₂ and R₃ are selected so that this elevated area16 is located within the evanescent coupling region of the core 12. By"evanescent coupling region", we mean the region of the fiber claddingsin which a significant portion of the wings or the evanescent field ofthe propagating optical signal in fiber 10 that exists outside its coreis found.

When a fiber is constructed with this profile, light with aRaman-shifted wavelength is radiated out of the core, but the desiredlasing wavelength is transmitted in the core as indicated in FIG. 1A.This effect can be enhanced by bending at an appropriate bend radiuswhich depends on the fiber parameters, materials and indices and isdetermined by applying an optical signal to the fiber and measuring bothRaman-shifted wavelength and the desired lasing wavelength (orfrequency) outputs. Then, the fiber is subjected to bending whilemonitoring these outputs to determine the best bend positioning wherethe Raman-shifted wavelength is minimal while the desired wavelength isoptimal.

This fiber configuration results in optical fiber 10 having an insertionloss characteristic at the Raman wavelength which is greater than theinsertion loss of conventional optical fibers at the Raman wavelengthbecause the elevated index area effectively enhances the sharpness ofthe wavelength cutoff due to bending losses of optical fiber 10.

Furthermore, when the core 12 and cladding layers 14-18 are providedhaving indices of refraction with the relative relationships shown inFIG. 1A, the insertion loss characteristic presented by fiber 10 tosignals propagating at the Raman wavelength is distributed along thelength of the fiber 10. The distributed loss effectively raises thepower threshold at which the stimulated Raman scattering occurs in thefiber 10. When such a fiber is used as a fiber laser, the power outputat the lasing frequency can be much higher before Raman shifting occurs,resulting in higher power output at the lasing frequency.

FIG. 2 is a cross-sectional diagram of another embodiment of a fiber inwhich a frequency selective construction is used. Referring now to FIG.2, an optical fiber 20 includes a core 22 provided, for example, fromdoped silica glass having an index of refraction typically of about 1.47and having a diameter typically of about 2-5 μm and doped withappropriate rare earth ions. Such rare earth ions constitute the activeelements in the optical fiber 20 and as such, dictate the lasingwavelengths at which gain is achieved in the case of a fiber laser.

A first cladding layer 24 disposed about core 22 encompasses the entirelongitudinal extent of the core 22. The first cladding layer 24 may beprovided, for example, from undoped silica having an index of refractiontypically of about 1.45. In this particular embodiment, the claddinglayer 24 is provided having a circular cross-section within a thicknesstypically of about 2-5 μm.

A second cladding layer 26 is disposed about the first cladding layer24. Cladding layer 26 is provided having a circular cross-section and isprovided having a thickness typically of about 2-5 μm. The secondcladding layer is formed from a suitable polymer such as a fluoropolymeror from a conventional low index glass having an index of refractiontypically of about 1.39. The second cladding layer is doped with amaterial having a relatively sharp absorption characteristic to signalshaving a wavelength corresponding to a Raman-shifted wavelength. Sincethe Raman-shifted wavelength is higher than the operating wavelength, itwill have a larger Gaussian spot size and larger evanescent wingsextending into claddings 24 and 26 compared to the operating wavelengthpropagating mode and, therefore, will be much more subject to theabsorptive effects of cladding 26. The absorptive cladding is alsoconstructed to have a low absorption to signals with the desired lasingwavelength. Absorptive cladding 26 may be illustratively fabricated fromsilica glass doped with rare earth or transition metal ions. Suitabledoping materials include Sm³⁺, Ho³⁺, Tm³⁺, Pr³⁺ and Dy³⁺.

Second absorptive cladding 26 is surrounded by pump cladding 28 whichhas a significantly larger diameter compared to the diameter of thecore/cladding region 22-26, For example, pump cladding may have a 250 μmdiameter. As known to those skilled in this art, pump cladding 28 mayalso have a different cross-sectional configuration from thecore/cladding section of fiber 10, such as square, rectangular orelliptical. Pump cladding 28 would be surrounded by an outerlight-confining, protective cladding.

By providing absorptive cladding 26 with a relatively high absorptioncharacteristic at the Raman-shifted wavelength, fiber 20 presents arelatively high insertion loss characteristic to energy propagatingalong the length of the fiber at the Raman shifted wavelength while alsopresenting a relatively low insertion loss characteristic to energypropagating along the length of the fiber 20 at the desired lasingwavelength. This loss is also advantageously distributed over the lengthof the fiber.

In still another embodiment schematically illustrated in FIG. 3, anoptical fiber of length L is constructed with a core with a long periodgrating 302 formed therein and located generally at the center of thefiber. The grating 302 is fabricated using conventional techniques witha periodicity selected to provide a relatively high insertion loss atthe Raman frequency while simultaneously providing a relatively lowinsertion loss at the lasing frequency. The long period grating 302 actsas a discrete loss element to Raman wavelength signals propagating inthe optical fiber. The grating 302 is generally tens to hundreds oftimes the lasing wavelength and causes the Raman-shifted wavelength tobe directed out into the outer cladding. This approach has the advantagethat it does not require a special fiber structure.

In some applications, it may be preferable to construct the fiber corewith a region having a plurality of long period gratings, 402-408,formed along the fiber length as schematically illustrated in FIG. 4.Since the gratings 402-408 act as discrete loss elements, severalgratings will have to be used along the fiber length, but this structurehas the advantage that the loss is more distributed over the length ofthe fiber. In this manner, Raman-shifted is not allowed to build upbefore being directed out of the fiber. The efficiency of the fiber 400is thus enhanced over the single gating structure illustrated in FIG. 3.

Referring now to FIG. 5, an neodymium (Nd) doped double clad fiber 30 isconstructed to operate at a desired fundamental wavelength of 940 nm. Aspreviously mentioned, one problem with operating the Nd doped fiber 30at 940 nm wavelength is that Nd is a three-level system at 940 nm andthe gain is much higher at a fundamental lasing wavelength of 1060 nmwhere Nd is a four-level system. Conventional operation at a wavelengthof 940 nm thus requires a relatively high pump intensity.

Fiber 30 has a conventional construction comprised of a core, an innercladding and at least one outer cladding. The core is single mode with amode diameter of approximately 8-10 microns. The inner cladding size ofthe fiber is approximately 150 microns by 300 microns (rectangular) forconventional double clad fibers. Other geometries of the inner claddingand sizes are also possible and well-known. Smaller inner cladding sizeswould be preferred for 940 nm operation, assuming the same pump powercan be launched in the fiber. The smaller cladding size would result ina higher pump intensity, an improved population inversion and associatedgain at 940 nm. Typical fiber length for a double clad fiber with 130 by360 micron inner cladding is 50 meters to allow for a 90 percentabsorption of the pump light.

In accordance with one aspect of the invention, the fiber 30 is coiledaround a mandrel 32 having a diameter typically of about 8 millimeter(mm). For a typical 50 m length fiber there would be about 500-700 turnson the mandrel. A pump source 34 provides a pump signal to a first endof fiber 30 to operate the fiber 30 at a laser wavelength of 940 nm.Bending the fiber at the 8 mm diameter causes radiation of light at awavelength of 1060 nm out of the fiber 30 while maintaining gain at awavelength of 940 nm. By inhibiting the gain of the fiber 30 at awavelength of 1060 nm, the occurrence of ASE at a wavelength of 1060 nmis prevented and the fiber 30 is operational at a wavelength of 940 nm.Due to the bend induced loss, the gain of the fiber at a wavelength of1060 nm is inhibited continuously along the length of the fiber.

The wavelength at which the light is radiated out of the fiber increaseswith a bend radius of a fiber and depends on the mode diameter in asingle mode core. By bending the Nd fiber with a bend radius typicallyof about 8 mm, the light at 1060 nm is radiated out of the fiber whilemaintaining gain at 940 nm. The particular bend radius which is used,however, depends upon a variety of factors, including, but not limitedto, the type of fiber, its parameters and indicies, the desired lasingwavelength, etc.

FIG. 5A illustrates generally how the transmission through a fiber isaffected by bending. Referring now to FIG. 5A, a typical plot ofwavelength versus insertion loss generated by transmission ofsubstantially white light through the core of a double clad fiber isshown. The measurement was made on a double clad fiber having a coredoped with ytterbium (Yb) (specifically doped with ytterbium ionstypically denoted as Yb³⁺) and coiled around a mandrel having a diametertypically of about 28 mm. The plot gives the measured emission (verticalscale) for the output of the fiber a function of wavelength (horizontalscale). The horizontal scale runs from 1000 nm to 1500 nm, correspondingto 50 nm per division.

As illustrated in the graph, at relatively short wavelengths, forexample, less than 1100 nm, the light is absorbed in the fiber core dueto absorption by the Yb ions producing the edge 310 illustrated in FIG.5. For wavelengths longer than 1250-1300 nm, the light is radiated outof the fiber due to bending induced loss, producing the edge 300illustrated in the curve. The important part of the graph is the bendinginduced loss edge 300. By bending the fiber more strongly, this bendedge 300 can be shifted to shorter wavelengths. Notice that the losschanges relatively slowly with the wavelength--about 10 dB over about a50 nm transition range.

A 50 nm wide transition region would be sufficient in the Nd dopedsystem, mentioned above, to eliminate gain at 1060 nm without affectingthe gain at 940 nm. Other conditions may require a sharper change inscatter loss vs. wavelength. For example, as mentioned above, Raman gainin fibers is about 50-70 nm. To suppress this gain, a sharper dependenceon wavelength may be required. By designing a fiber with the indexprofiles discussed above in combination with bending, it is possible toincrease the slope in this area in order to help eliminate thepossibility of developing Raman gain. An increased slope would result ina better discrimination between two adjacent wavelengths which bothexperience gain in the fiber.

The embodiments of this invention are not limited to circular fibers, asthe core and/or inner pump cladding may have other geometricalcross-sections as is known in the art, such as rectangular or ellipticalcross-section contours. Also, the optical fibers may includepolarization preserving structural features or be polarizationinsensitive.

Having described preferred embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may be used. Therefore, theinventive concepts are not limited to disclosed embodiments, but ratherare limited only by the spirit and scope of the appended claims.

What is claimed is:
 1. An optical fiber system having an optical gainmedium in which optical energy is generated at both a desired wavelengthand an undesired wavelength, the system comprising:a doped fiber corethrough which the desired wavelength and the undesired wavelengthpropagate, the undesired wavelength having larger evanescent wings thanthe desired wavelength; and an absorbing material located radiallyoutward from the core, the absorbing material being separated from thecore at such a distance that the evanescent wings of the undesiredwavelength extend significantly into the absorbing ring, while theevanescent wings of the desired wavelength do not extend significantlyinto the absorbing ring.
 2. An optical fiber system as recited in claim1 wherein the absorbing material is doped with rare earth ions.
 3. Anoptical fiber system as recited in claim 1 wherein the absorbingmaterial is doped with transition metal ions.
 4. A system as recited inclaim 1 further comprising a spacing layer between the core and theabsorbing material that contributes to the separation between the coreand the absorbing material.
 5. A system as recited in claim 4 whereinthe spacing layer has a thickness of approximately 2-5 nm.
 6. A systemas recited in claim 4 wherein the spacing layer has an index ofrefraction lower than that of the core and higher than that of theabsorbing material.
 7. A system as recited in claim 4 wherein thespacing layer has an index of refraction lower than that of the core andthat of the absorbing material.
 8. A system as recited in claim 4wherein the spacing layer has been modified to enhance the spreading ofevanescent wings with higher wavelength.
 9. A system as recited in claim4 wherein the spacing layer is constructed from a material having anominal index of refraction and wherein the spacing layer has been dopedto reduce its index of refraction below said nominal index ofrefraction.
 10. A system as recited in claim 1 wherein the opticalenergy at the undesired wavelength is generated at least in part byRaman gain.
 11. A system as recited in claim 1 wherein the opticalenergy at the undesired wavelength is generated at least in part byamplified spontaneous emission.
 12. A system as recited in claim 1wherein the core is a single mode core.
 13. A system as recited in claim1 wherein the core is doped with rare earth ions.
 14. A system asrecited in claim 1 wherein the undesired wavelength is longer than thedesired wavelength.
 15. A system as recited in claim 1 furthercomprising a pump cladding that surrounds the core and receives opticalenergy suitable for optically pumping the core.
 16. A system as recitedin claim 1 wherein the absorbing material is significantly moreabsorbent at the undesired wavelength than at the desired wavelength.17. A system as recited in claim 1 wherein the system comprises a fiberlaser.
 18. A system as recited in claim 1 wherein the system comprisesan optical amplifier.
 19. A cladding pumped optical fiber systemcomprising:a single mode core doped with rare earth ions in which aregenerated a desired wavelength and an undesired wavelength; a pumpingsystem that generates pumping energy and delivers the pumping energy tothe core; an spacing layer surrounding the core; and an absorbingmaterial that is located radially outward from the spacing layer andthat is significantly absorbent at the undesired wavelength, theabsorbing material being separated from the core such that evanescentwings of an undesired wavelength in the core extend significantly intothe absorbing material, while evanescent wings of a desired wavelengthin the core do not extend significantly into the absorbing material. 20.An optical fiber laser as recited in claim 19 wherein the absorbingmaterial is doped with a dopant that includes rare earth ions.
 21. Anoptical fiber laser as recited in claim 21 wherein the absorbingmaterial is doped with a dopant that includes transition metal ions. 22.A system as recited in claim 19 wherein the system comprises a fiberlaser.
 23. A system as recited in claim 19 wherein the system comprisesan optical amplifier.
 24. An optical fiber laser as recited in claim 1wherein the absorbing material is substantially radially uniform about alongitudinal axis of the fiber.
 25. An optical fiber laser as recited inclaim 19 wherein the absorbing material is substantially radiallyuniform about a longitudinal axis of the fiber.